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HGDT Max Bar Pres > Cham Pres

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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HGDT Max Bar Pres > Cham Pres

Unread postAuthor: saefroch » Tue Nov 27, 2012 7:52 pm

This is the hoped configuration for my current build. Why is the max barrel pressure much greater than the max chamber pressure?

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Unread postAuthor: D_Hall » Tue Nov 27, 2012 9:48 pm

Unburned fuel/air is being pushed into the barrel and then being ignited IN the barrel.
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Unread postAuthor: saefroch » Tue Nov 27, 2012 9:52 pm

So this is accurate?

Based on the graph of barrel pressure over time (I can't even see the peak that goes up to 6.5kpsi), will the barrel's inertia take care of this extra pressure?
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Unread postAuthor: D_Hall » Tue Nov 27, 2012 10:02 pm

I don't know if it's *accurate*. It's just the best I could do with the knowledge and such I had available. But you aren't looking at a bug. You're looking at the results of significant amounts of unburned fuel/air flash igniting in the barrel. And yes, much of that pressure simply bleeds backwards into the combustion chamber.


As for the inertia taking care of it... It may. Hard to say. A lot of variables here... But personally, I wouldn't trust it without looking at the situation VERY closely.



edit: Actually, I know that it's not *accurate*. The model assumes that once the flame front makes it to the barrel that it will instantaneously ignite everything in the barrel. Clearly this is a worst case scenario. Truth is that the propagation in the barrel isn't going to exceed 1800 m/s. How fast WILL it propagate? I'm unsure, but based on everything I've seen with VERA, it will be *damned fast*. The instantaneous model gives 6000 psi? I wouldn't be surprised to see 4000 psi actual.
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Unread postAuthor: saefroch » Tue Nov 27, 2012 10:42 pm

I recall previous discussion about there eventually being a 1-D version of HGDT, and issues with GGDT near-supersonic. Yeah, I wasn't asking if it was true to the real world, but if I hit upon a bug.

I'll keep your caution in mind. I believe DYI ran some numbers for me a while ago that put the pressure ceiling of the parts I'm using for the barrel at 13.4kpsi or so, so I should be safe for at least the first few operations.

Thanks for the assistance!
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Unread postAuthor: D_Hall » Tue Nov 27, 2012 11:01 pm

Yeah, I've got the program architecture and methodology mapped out in my mind for a new program that will combine GGDT and HGDT into one program utilizing CFD for better results. Unfortunately, live has been insane as of late and I've not been able to pursue actual implementation. Sure, I get chunks of time here and there, but to do what I want I'm going to need large blocks of time and that just isn't happening!
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